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PHILADELPHIA (CBS) – The heat is here, and it’s here to stay for the rest of the week. Once we reach the 90 mark tomorrow, this will officially be the fourth heat wave of the year, and it looks like it will run seven days this time, each one hotter than the last.

A break in the cloudy and rainy pattern has been traded for sunshine and very warm temperatures. An area of high pressure currently situated in the Ohio Valley is providing plenty of sunshine and very few clouds for the region. The sunshine will allow our daytime temperatures to skyrocket. Scorching temperatures combined with high dew point values will make for very uncomfortable and oppressive-feeling days to come.

Philadelphia and the surrounding counties are under an Excessive Heat Warning through Wednesday evening. Temperatures will remain in the mid to upper 90’s through the work week into the weekend. Heat indices will climb as dew points and relative humidity values increase. This will cause temperatures to feel as if they are anywhere from 100-103 degrees during the height of the day. Thursday and Friday will bring even higher heat index values, soaring to 105 degrees.

Precautions must be taken during this period of excessive heat. Any outdoor activities should be limited or accompanied by frequent breaks in the shade and plenty of hydration. Heat illness is likely and can be avoided by staying in air conditioned buildings and staying hydrated. In Philadelphia, many homes are made from brick that may be older and not have central air conditioning. Brick can heat up very quickly and retain heat. If you plan to use a fan, make sure to do so with the windows open.

Nighttime temperatures will not act as a relief for those in urban areas. Lows are forecasted to stay in the upper 70s to about 80 degrees. Dew points into the nighttime will make for sticky and muggy evenings throughout the week.

Much of the week is expected to be dry, except for very scattered pop up showers due to daytime heating and humidity. Any rainfall will bring very short relief until the weekend, when a break in the heat is expected. Much cooler and more comfortable summertime temperatures will follow a cold front on Sunday, with the possibility for showers and thunderstorms in advance of the front.